Rhino Mocks: Can I use Stub() when one of my parameters is Expression<Func<T1, T2>>?

Question

I have a method on an interface that looks like this and I want to stub it with Rhino Mocks:

TValue GetPropertyOfExistingObject<TValue>(long id, Expression<Func<T, TValue>> propertyExpression);

My code that does the stubbing looks like this:

var service = MockRepository.GenerateStub<IQuoteService>();
service.Stub(s => s.GetPropertyOfExistingObject(1, q => q.QuoteNumber)).Return(1234);

Notice that one of the parameters in that method is an Expression<Func<T1, T2>>, and this stub is not returning the specified value. I know I can do this by using WhenCalled() but I was wondering if Stub() should work with expression parameters or if I'm just doing something wrong.

Answer 1

I think the problem is related to how Expressions test equality. I just did a quick test in Snippet Compiler and my expressions never evaluated as the same:

 Expression<Func<int, string>> p = i => i.ToString();
 Expression<Func<int, string>> s = i => i.ToString();
 var b = p.Equals(s) || p == s;

(b was false for this test)

Probably in order for your test to work as is, you'd have to ignore the actual value of the second parameter (which may or may not be acceptable; If unacceptable I think you'll have to go the WhenCalled route).

Answer 2

You can create a method that evaluates the equality between two expressions:

public class ExpressionMatcher
{
    public static Expression<Action<T>> Matches<T>(Expression<Action<T>> action)
    {
        var methodName = ((MethodCallExpression) action.Body).Method.Name;
        return Arg<Expression<Action<T>>>.Matches(a => ((MethodCallExpression)a.Body).Method.Name.Equals(methodName));
    }
}

Then change your stub statement to wrap the expression in a call to the expression matcher:

service.Stub(s => s.GetPropertyOfExistingObject(Arg<int>.Is.Equal(1), ExpressionMatcher.Matches<Quote>(q => q.QuoteNumber))).Return(1234);